In an electronic data storage and retrieval system, a magnetic recording head typically includes a reader portion having a sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the sensor, which in turn causes a change in the electrical properties of the sensor. The sensing layers are often called free layers, since the magnetization vectors of the sensing layers are free to rotate in response to external magnetic flux. The change in the electrical properties of the sensor may be detected by passing a current through the sensor and measuring a voltage across the sensor. Depending on the geometry of the device, the sense current may be passed in the plane (CIP) of the layers of the device or perpendicular to the plane (CPP) of the layers of the device. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover information encoded on the disc.
A structure in contemporary read heads is a thin film multilayer structure containing ferromagnetic material that exhibits some type of magnetoresistance (MR), such as tunneling magnetoresistance (TMR). A typical TMR sensor configuration includes a multilayered structure formed of an insulating thin film, or barrier layer, positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer, or between two ferromagnetic free layers. The barrier layer is thin enough to allow electron tunneling between the magnetic layers. The tunneling probability of an electron incident on the barrier layer from one magnetic layer depends on the character of the electron wave function and the spin of the electron relative to the magnetization direction in the other magnetic layer. As a result, the resistance of the TMR sensor depends on the relative orientations of the magnetization of the magnetic layers, exhibiting a minimum for a configuration in which the magnetizations of the magnetic layers are parallel and a maximum for a configuration in which the magnetizations of the magnetic layers are anti-parallel.
Some TMR sensors incorporate a barrier layer including magnesium oxide (MgO), which allows for magnetoresistive ratios approaching 200% when the resistance-area (RA) product of the TMR sensor is high (i.e., greater than 10 Ω·μm2). In order to achieve a high magnetoresistive ratio, a CoFeB based single free layer adjacent to the insulating thin film is sometimes used in conjunction with a high temperature anneal (i.e., greater than 300° C.). However, this device results in a high positive magnetostriction, a high RA product, and a high temperature anneal, all of which either reduce performance of the device or make processing of such a device difficult. To reduce the magnetostriction of the device, the CoFeB free layer may be laminated or co-sputtered with a material having a negative magnetostriction. While this results in a sensor having low magnetostriction, the magnetoresistive ratio of the sensor is significantly reduced.